In the manufacture of printed circuit cards and boards, a dielectric sheet material is employed as a substrate. Conductive patterns are provided on one or both the major surfaces of the substrate. In some applications more than one level of conducting circuit patterns are required on one or both sides of the substrate. Plating is the most commonly used method of metalizing these substrates. Generally, in order to plate on the substrate it must be seeded or catalyzed prior to the deposition of metal thereon. Included among the various dielectric materials suggested for substrates are various organic polymers including polyimides, polyesters, polysulfones and the like. Moreover, multilayer dielectric structures, including multilayer polyimide structures, are used as the top wiring levels of integrated circuit chips and VLSI packaging. Plating techniques can be used to fabricate these wiring levels.
Among the more widely employed procedures for catalyzing a substrate is the use of a stannous chloride sensitizing solution and a palladium chloride activator to form a layer of metallic palladium particles thereon. For instance, one method of catalyzing a dielectric substrate is exemplified by U.S. Pat. No. 3,011,920 which includes sensitizing a substrate by first sensitizing it with a solution of a colloid metal, activating the colloid with a selective reagent to remove unreactive regions on the sensitized dielectric substrate, and then electrolessly depositing a metal coating on the sensitized substrate, for example, copper can be deposited from a solution of a copper salt and a reducing agent.
To fabricate a multilayer structure having a plurality of conducting layers using a stannous chloride sensitizing solution in a palladium chloride activator solution requires many steps. For example, on a first dielectric layer there is deposited a second dielectric layer; a pattern is formed in the second dielectric layer; and a seed material is deposited only onto the exposed region of the first dielectric layer. In order to achieve this selective seeding, the surface of the second dielectric layer must be protected to prevent the seeding thereon. This can be achieved, for example, by disposing on the second dielectric layer a patterned photoresist material. The requirement to use this photoresist material adds complexity and cost to the fabrication process and is also a potential source of undesired contamination.
According to one aspect of the present invention a first dielectric layer of an electroactive material is provided. A second dielectric layer of a different electroactive material is disposed over the first dielectric layer. The first and second electroactive material have differing redox potentials. Electrons are provided at an energy sufficient to reduce only one of the electroactive materials. The structure is then placed into contact with a seeding solution that contains cations of a metal. Electrons are transferred from a reduced electroactive material to the cations which deposit onto the reduced electroactive material as seed in the zero oxidation state. Next, a metal can be deposited from an electroless or electroplating bath onto the seed metal.
It is an object of this invention to selectively reduce a first electroactive material in the presence of a second electroactive material wherein the first electroactive material has a redox potential positive of the redox potential of the second electroactive material.
It is another object of this invention to provide a multilayer structure having layers of material of different redox potentials.
Another object of this invention is to provide structures having layers of electroactive material with different redox potentials containing electrically conducting patterns embedded therein.
Electrons are supplied to the redox sites of the electroactive material by either means of a cathode in an electrochemical circuit or from a reducing agent in solution. The electrons supplied by the electrode or reducing agent have a potential negative of the potential of the electroactive material on which the electrons are being deposited but more positive of the reduction potential of the electroactive material on which the electrons are not deposited.
U.S. Pat. No. 3,505,168 to Dunphy et al. describes a heat sealable laminate structure having at least two layers of polyimide polymeric material wherein the polyimide of one of the layers is different from the polyimide of the other layers. There is no teaching or suggestion therein to dispose a pattern in either one of the polyimide layers nor to form an electrical conductor in the pattern in the polyimide layer.
U.S. Pat. No. 4,788,098 to Sado et al. describes an aromatic polyimide laminate sheet wherein a second film layer is preferably the same as the first film layer but can be different. There is no teaching or suggestion to form a pattern in one of the polyimide layers nor to form a conducting pattern within a pattern in the polyimide layer.
U.S. Pat. No. 4,347,286 describes compound films of a polyimide film and a heat fusible layer of a polyimide precursor. There is no teaching or suggestion to form patterns in one of the films nor to form a conducting pattern within a pattern in a dielectric film.
U.S. Pat. No. 4,710,403 to Krause et al. describes a method of metallizing an electroactive center-containing polymeric substrate by first supplying electrons to the electroactive center by contacting the substrate with a solution containing ions, such as intercalation ions, e.g., Te.sup.2-, V(II) and Co(I) complexes in aqueous or methanolic reductants. The reduced polymer is then contacted with a solution of metal salts to result in deposition of the metal. There is no teaching or suggestion in Krause et al. to selectively reduce one electroactive center-containing polymeric material in the presence of a second electroactive center-containing polymeric material.